1. Field of the Invention
The present invention refers to a method of producing a micromechanical relay.
2. Description of Prior Art
A relay fundamentally serves to switch electric currents. Since the switching of electric currents often has to be accomplished in the field of technology, there is a large field of application for relays. Recently, micromechanical relays have been developed, which, due to the use of semiconductor technology, are based on a new electrostatic principle of action.
This electrostatic principle of action permits an almost powerless switching of currents. This property is of importance especially in cases of use where a connection to the electric mains is not possible, i.e. in cases of use where power is supplied by a battery. Such use becomes increasingly common, e.g. in wireless transmission in the field of communication technology. In order to guarantee here a sufficiently long operating time, it is important to keep power consumption low. A micromechanical relay can expediently be used in this respect.
A second exemplary field of use for a micromechanical relay is the switching of high-frequency signals. Such a high-frequency relay must have a low characteristic impedance so that it can be used e.g. in the field of high-frequency measurement technology. A micromechanical relay has this property so that such a component offers excellent advantages especially as far as high-frequency technology is concerned. DE 4205029 C1 and DE 4437261 C1, for example, disclose electrostatically actuated micromechanical relays. The electrostatic forces are in each case produced according to the same principle. A voltage is applied between two capacitor plates. A micromechanical structure used as a movable counterelectrode changes its position relative to a fixed electrode due to the electrostatic force. When a voltage is applied to the electrodes, the movable counterelectrode is therefore attracted by the fixed electrode.
For producing the micromechanical relay, a freestanding movable structure carrying the contact pieces must be produced. This is done by means of a back etching process in the case of the known embodiments. In such a back etching process, the wafer is etched through from the back in a KOH etching bath until a freestanding structure is obtained. Due to the specific etching angles used in this process, the space required for a structure is much larger than the structure itself. It follows that, in the case of the known manufacturing method for micromechanical relays, the area required per component during the production process is much larger than the area which is actually occupied by the finished component.
It is true that, when a movable structure has been etched free by means of a back etching process according to the known method, a freestanding structure is obtained, but a substrate material which could be used as an electrode no longer exists below said freestanding structure, i.e. the movable beam. Hence, at least a second chip provided with a fixed electrode must be arranged above the processed chip to which a voltage can then be applied. The connection of the two chips, the so-called chip bonding is, however, very expensive and difficult.
In the case of all known micromechanical relays, the freestanding beam carries the contact structures, one possibility being to arrange this contact structure such that it extends in the longitudinal direction of the beam. This has, however, the disadvantage that this metal structure must be very thin so as to reduce the thermostatic bimetal effect between the supporting beam and the conductor path. It is therefore common practice to produce micromechanical relays with a dual contact in the case of which a contact bridge extends transversely across the movable beam top. On the basis of this arrangement, the beam is prevented from curving due to the thermostatic bimetal effect.
U.S. Pat. No. 4,959,515 discloses a method of producing a micromechanical switch provided with a lever arm which is fixed at one end and the free end of which can be used for closing a contact by means of electrostatic attraction. The lever arm fixed at one end is produced by fully etching a nickel layer, which is used as a sacrificial layer, by means of a strong solution.
EP-A-0711029 describes the production of a microstructure with a movable arm by etching off an adhesive-agent sacrificial layer below said movable arm.